1. Field of the Invention
The present invention relates to a storage control sub-system comprising virtual storage units.
2. Description of the Related Art
For example, in a storage system for trunk business use, which handles a large volume of data, a storage control sub-system constructed separately from the host computers (below, referred to as “host terminals”) is used to manage data. This storage control sub-system comprises, for example, a RAID (Redundant Array of Independent Inexpensive Disks) constituted by providing a plurality of disk type storage devices in an array fashion.
For example, Japanese Patent Laid-open-No. (Hei) 9-288547 discloses a system wherein a plurality of physical devices aligned in series are provided as a single virtual device (paragraph 23).
Furthermore, for example, Japanese Patent Laid-open No. (Hei) 11-345158 discloses a method for acquiring a differential dump for a pair of principal and subsidiary volumes which respectively record updates to the principal and subsidiary volumes in respective volume update information, whereby the update information for said subsidiary volume is made to coincide with the update information for said principal volume, when shifted to a mirror state, and when the dump is started, the mirror state is changed to a temporary mirror release state, and the update information for the principal volume is initialized, in addition to which a dump of the updated portion is obtained from said subsidiary volume.
There are storage control sub-systems wherein, for example, of a plurality of logical storage devices (hereinafter, referred to as “LDEV” as an abbreviation of “Logical Device”) prepared on one or a plurality of disk type storage devices, a single storage unit constituted by one or more the LDEVs is provided to a host terminal as a LU (Logical Unit). In a storage control system of this kind, for example, an LU pair may be formed wherein one LU is taken to be a primary LU and another LU is taken to be a secondary LU, and the data in the primary LU prior to update may be copied to the secondary LU (in other words, a so-called “snap shot” may be created).
In general, the respective LUs forming the LU pair have the same storage capacity. Therefore, if the storage capacity of the primary LU is large, then an LU having a large storage capacity is also required for the secondary LU. However, in the aforementioned snap shot, the data size of the data prior to update that is to be copied may be small. Therefore, there may be cases in which the storage capacity of the secondary LU is made unnecessarily large.
Furthermore, for example, if the storage control subsystem receives a read capacity command from the host terminal, then it may report the storage capacity of the LU in the storage control sub-system, to the host terminal. Here, if the storage control sub-system reports a different storage capacity to a previously reported storage capacity, with respect to the same LU, then there is a possibility that confusion may result in the host terminal.